A Western Pennsylvania House seat that backed President Donald Trump by 20 points became the latest special-election battleground this week.

After months of shadow-boxing in the otherwise solidly Republican district, investments this week by both the DCCC and the NRCC signal the race between Democrat Conor Lamb and Republican Rick Saccone is more competitive than recent history would suggest.

The NRCC has reserved more than $1 million on ads on broadcast and cable TV stations to boost Saccone in the race to replace former Rep. Tim Murphy (R-Pa.). Murphy resigned last October, after reports surfaced that the anti-abortion lawmaker had an extramarital affair and allegedly encouraged his lover to terminate a pregnancy. The DCCC’s investment is smaller, but both committees are signaling that they would make major investments if the race is close between now and the March 13 election.

The NRCC’s ad reservations begin on Monday and run all the way through March 13. But a source familiar with the buy said that the NRCC expects to spend more over that period than what’s currently on the books.

Several other Republican outside groups are also involved — including Congressional Leadership Fund, the 45Committee and Ending Spending, a group founded by the mega-donor Ricketts family. Congressional Leadership Fund alone has reserved $1.6 million in ads, beginning Friday.

Trump has already appeared in the district, endorsing Saccone and calling him “special” during an official speech billed as a discussion of the economy.

Democrats stayed out of the fray until this week, when the DCCC reserved $236,000 on TV ads for just a two-week flight, beginning next Tuesday.

The race in Pennsylvania’s 18th Congressional District is the seventh House special election since 2017 — Republicans held five seats vacated by GOP members, while Democrats retained their only open seat — but just the third in which the DCCC has actively competed. The party spent more than $5 million to help boost Democrat Jon Ossoff’s unsuccessful campaign in last June’s special election for a suburban Atlanta House seat and made a more modest, $380,000 investment in an at-large Montana seat. GOP candidates retained those seats by narrow margins.

The NRCC, meanwhile, has been more active. It spent nearly $7 million in Georgia, $1.8 million in Montana and made low-six-figure expenditures in ruby-red districts in Kansas and South Carolina that the party also held.

The Pennsylvania seat, which covers much of the southwestern corner of the state, is an uphill climb for Democrats. Both Trump and 2012 GOP nominee Mitt Romney won 58 percent of the vote there. Murphy ran unopposed in both 2016 and 2014; he won 64 percent of the vote in 2012, when he had a Democratic opponent.

Lamb, a former federal prosecutor, is trying to cast himself as a bipartisan problem-solver who is critical of both parties. Earlier this month, Lamb told The Pittsburgh Post-Gazette that he would not support Nancy Pelosi as the Democratic leader, citing a “need [for] new leadership on both sides.”

That sentiment is echoed in his campaign’s TV ads. “In the military, as you know, when you don’t get the job done, you get relieved,” Lamb, a Marine Corps veteran, said in a recent TV ad. “We need more of that in Washington.”

A Democratic poll for a labor union conducted last week showed Saccone with a 3-point lead, a slim margin for a district that backed Trump by 20 points in 2016. Trump’s approval rating in the district has also dropped to 49 percent, roughly equal to the 47 percent who disapprove of his job performance.

A Democratic operative familiar with polling on the Pennsylvania race said the margin remains in the mid-single digits.

But Republicans believe they can paint Lamb as a puppet of Nancy Pelosi, a strategy the NRCC and Republican outside groups deployed to great effect in Georgia’s special election to boost Republican Rep. Karen Handel over Democrat Jon Ossoff.

“His name is Conor Lamb, but in Washington, he’d be one of Nancy Pelosi’s sheep,” a recent CLF TV ad says, digging at the Democrat’s last name. “Lamb would join the liberal flock and follow Pelosi’s lead, voting the straight liberal party line for Pelosi’s extreme agenda.”

Local and national Republicans are concerned about Saccone’s fundraising, as the state legislator raised just more than $70,000 in eight months while running for U.S. Senate earlier this year, before switching to the House race. The White House political director Bill Stepien “expressed displeasure” about Saccone’s lackluster fundraising totals in early January, POLITICO reported.

Saccone and Lamb won’t be required to file fundraising reports until Jan. 31, but operatives in both parties warned that the totals covered the candidate selection in late November through the holiday season, a slow fundraising time. Lamb got an endorsement from the Daily Kos, a liberal blog, and other progressive groups, which will help channel online, small-dollar donors to his campaign in the new year.

The winner of the special election will serve out the remainder of Murphy’s term but would have to run again in November to secure a full, two-year term. But that is likely to be in a reconfigured district: The state Supreme Court ruled earlier this week that the existing congressional map was gerrymandered to favor Republicans and violates the state constitution. Though the state Supreme Court said it would allow the March special election to go ahead under the now-invalidated district lines.

Republicans have asked the U.S. Supreme Court to set the ruling aside and let the 2018 elections go on under the new map. But if that effort fails, either the legislature or the court itself will draw a new map, with unknown changes to Pennsylvania’s 18 House districts.